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Durability of Alkali-Resistant Glass Fibers Reinforced Cement Composite: Microstructural Observations of
Degradation
Nourredine Arabi, Laurent Molez, Damien Rangeard
To cite this version:
Nourredine Arabi, Laurent Molez, Damien Rangeard. Durability of Alkali-Resistant Glass Fibers Reinforced Cement Composite: Microstructural Observations of Degradation. Periodica Polytechnica Civil Engineering, he Faculty of Civil Engineering of the Budapest University of Technology and Economics, In press, pp.10631. �10.3311/PPci.10631�. �hal-01712823�
Durability of Alkali-Resistant Glass
Fibers Reinforced Cement Composite:
Microstructural Observations of Degradation
Nourredine Arabi1*, Laurent Molez2, Damien Rangeard2
Received 14 February 2017; Revised 02 October 2017; Accepted 05 January 2018
1 Materials Geomaterials and Environment Laboratory,
Badji Mokhtar Annaba University, BP 12 Annaba 23000, Algeria 2 Civil Engineering Materials and Mechanical Engineering Laboratory,
INSA Rennes, France
* Correspondent author, email: nourredine.arabi@univ-annaba.dz
OnlineFirst (2018) paper 10631 https://doi.org/10.3311/PPci.10631 Creative Commons Attribution b research article
PPPeriodica Polytechnica Civil Engineering
Abstract
Usually, glass fibers in concrete permit the increase of the flexural strength. But the fibers in contact with cement are quickly degraded by alkali reactions due to the presence of portlandite. This article presents the results of investigations carried out to study the influence of curing conditions on the durability of alkali-resistant glass fibers in a cement matrix.
Test results showed that alkali resistant fibers treated with zirconium oxide, present the same degradation phenomenon.
The used cement nature has a large influence on the protec- tion of the fibers. In addition, the degradation is weakened when silica fumes are added, owing to the pozzolanic effect that decreases the portlandite quantity. In spite of those dif- ferent ameliorations, the use of such materials in humid envi- ronment is proving to be risky. The used techniques to study those phenomena are SEM equipped with energy dispersive spectrometer (EDX) to observe the attack microstructures of the fibers and X-ray diffraction to quantify the portlandite and other phase development.
Keywords
glass fiber; cement matrix; durability; curing conditions
1 Introduction
The use of fibres to strengthen materials which are much weaker in tension than in compression goes back to ancient times. Now, fibres of various kinds are used to reinforce a number of different materials, such as epoxies, plastics, ceram- ics and concrete [1, 2]. Glass fibers have particularly high an ultimate tensile strength and a very high Young’s modulus, compared to other organic or inorganic fibers [3]. As a solu- tion, they are a material of choice for improving the cracking resistance and the toughness of many composite products [4].
However, the studies on the Glass Fiber Reinforced Cement (GFRC) showed the low durability of the fibers in the cement matrix [5–7]. The fiber degradation is due to the portlandite (Ca(OH)2) produced during the hydration and the setting of the cement. This lime keeps the high pH of interstitial solution in the pores of cementitious matrix (pH of about 12.6). This alka- line environment leads to a rapid deterioration process which involves strength and weight losses, and a reduction in the fil- ament diameter [8]. This process can be attributed to breaking of the Si−O−Si bonds in the glass network, by the hydroxyl ions (OH−) which are highly concentrated in the alkaline pore solution as shown in Eq. (1):
| | |
− Si−O−Si + OH− Si− OH + SiO− (in solution) (1) | | |
In fact, portlandite increases the alkalinity of the matrix and develops large crystals. In addition, the glass fibre degrada- tion is also caused by the deposition and growth of the port- landite (Ca(OH)2). This portlandite fills spaces between and around the glass fibre filaments, thus cementing them together and reducing their flexibility. This causes excessive bonding and local concentration of stresses under load at the surface of fibres, resulting in embrittlement of GFRC [9]. Both phenom- ena weaken the fibers [8]. Alkali resistant glass fibers, called CemFil, have been developed using zirconium oxide (ZrO2) additives that inhibit the cement matrix attacks. However, they are still sensitive to a humid environment: a decreasing of the flexural strength is observed [10, 11].
Several techniques are used to protect fibers. One of these is to protect the fibers by using a cement at low basicity [12–16].
Some authors used a cement at low alkalinity such as the sul- phoaluminate cement (LASC) [17]. For this cement matrix no Ca(OH)2 crystals are detected by SEM observation and XRD analysis. The pH value of the pore solution for cement matrix of LASC is 10. Another technique can be also used by densi- fying the interface between fibers and matrix with polymers (PVA, AC or VAC) to prevent the lime diffusion into fibers [18, 19]. Other techniques consist in limiting the alkali-silica reaction by reducing the amount of portlandite into the matrix under pozzolanic effect. For instance, fly ashes, silica fumes or granulated blast furnace slag could be added to conventional concrete in order to substitute a part of the cement. [20–24].
Our study aims at analyzing the fibers evolution into the cement matrix, under different storage conditions and with or without silica fume addition. The fibers are observed by scanning electron microscopy (SEM) and the content of port- landite is determined by X-ray diffraction using the integrated method (semi-quantitative measurements of “step to step”).
2 Experimental 2.1. Materials
2.1.1. Matrix materials
The used cement is CEM II/B-V (32.5R) from Ciment Lafarge Ltd (France), conformed to EN 196-1 [25]. The chem- ical composition according to the manufacturer is given in Table 1. The sand used is a siliceous fraction that does not exceed 500 µm in size (see Fig. 1). Silica fume (a byproduct of the semiconductor industry brought from Bretagne area in France) added for a series so as to reduce the amount of port- landite by pozzolanic effect, it replaces cement (Table 4). To ameliorate the fresh mixture workability, the superplasticiser Sikament FF86, conformed to NF EN 934-2 [26], is used. Its physical and chemical characteristics are given in Table 2.
Fig. 1 Particle size distribution of sand
Table 2 Physical and chemical characteristics of the superplasticiser Sikament FF86
Aqueous solution
Density 1.23
pH 7–11
Cl– (weight %) < 0.1
Na2O equivalent (weight %) < 6.0
2.1.2. Glass fibers AR
The studied glass fibers are alkali resistant fibers, CemFil, from Saint Gobain Vetrotex manufacturer. The fibers (12 mm length) have a high content of zirconium oxide ZrO2 (16.6 mass
%), that should protect the fibers from the alkaline attacks of the cement matrix. The major physical and mechanical proper- ties, given by the manufacturer, are displayed in Table 3.
Table 3 Physical and mechanical properties of the AR fibers
Density 2.68 g/cm³
Tensile strength: virgin filament 3000 MPa
Tensile modulus 73000 MPa
Elongation at break 4.30%
Coefficient of linear thermal expansion (between 20
and 100°C) 5.10–6 m/m/K
Flammability 773 °C
2.2. Sample preparation and tests
Three series of mortars have been prepared. The formula- tion parameters of the different glass fiber reinforced matrix are shown Table 4. The low silica fume addition is condi- tioned by the use of CEM II/B 32.5 cement that contains 24 % of fly ash.
Table 4 Compositions of the used mortar series Constituents (weight %)
and ratios
CEM II/B-V( 32.5R)
1 (Reference) 2 3
Cement/Sand 1 1 1
Water/Cement 0.35 0.35 0.35
CemFil fibers 0 2 2
Silica fumes 0 0 3
Superplasticizer 0 0 2
Mortar samples (4 × 4 × 16 cm3) have been prepared and demoulded after 24 hours. They were cured in a moist room (20 °C and 95 % relative humidity) during 7 days and then during 1 year in different environments: in a moist room (20°C, 95 % relative humidity) and at ambient air. The sample formulations and curing condition are given Table 5.
Table 1 Chemical composition (mass %) of the cements and of the silica fumes.
Elements SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 Cl– Insolubles Loss on ignition
Cement 25.05 8.6 3.5 55.4 0.9 1 0.15 2.7 0.022 11.8 2.2
Silica fume 97.64 0.1 0.14 0.15 0.39 0.14
Table 5 Samples compositions and curing conditions Samples Fiber rates
(mass %) Silica fumes
(weight %) Storage conditions
FSH2 2 - Moist room
FCA2 2 - Ambient air
FSH3 2 3 Moist room
FCA3 2 3 Ambient air
The phase composition of each sample was determined by X-ray diffraction (XRD) using a Philips PW3710 type (Cu-Kα radiation) and the microstructure was examined using a scan- ning electron microscope SEM (JOEL JSM-6301F) coupled with EDX microanalysis.
3 Results and discussion
3.1. Quantitative analysis of Portlandite in cement matrix
According to some authors [9, 10] the presence of portlandite between fibers would be one of the causes of their degradation.
The calcium hydroxide is formed into the spaces between fibres as shown in Figure 2. Two modes of attack on the glass fibres are evident: hydroxylation and notching by Ca(OH)2 crystals.
The surface of fibres becomes generally hydroxylated due to the high pH of the matrix, but notching attack is concentrated at points where a fibre is in good contact with Ca(OH)2 crys- tals, leading to their impingement on and into fibres [27]. The content of portlandite was determined using X-ray diffraction.
Fig. 2 Filling of the spaces between the filaments by calcium hydroxide.
Note surface corrosion and notching by Ca(OH)2
The X-ray diffraction (λKα Cu, Ni filter), step-to-step) has been performed on 4 samples: FSH2 and FSH3 cured in moist room, FCA2 and FCA3 cured at ambient air. The X-ray pat- terns are given in Fig. 3. The peak intensity of the calcium hydroxide (portlandite) on the diffraction pattern is measured by the integrated method (semi-quantitative measurements of
“step to step”); results are reported in Table 6. It is also men- tioned the peak intensity of the calcium carbonate (calcite) in order to assume the carbonation of the samples.
Table 6 X-ray diffraction peaks intensity of the calcium hydroxide and the Calcite
2θ d(Å) Formulations
FSH2 FSH3 FCA2 FCA3
Calcium hydroxide [Ca(OH)2]
18.08 4.90 995 504 603 171
34.1 2.62 305 202 291 146
47.13 1.93 125 103 114 62
Calcite [CaCO3]
23.11 3.84 65 87 70 48
29.45 3.02 542 728 639 517
47.62 1.90 67 95 69 61
The intensity of the portlandite peaks is lower for the FCA samples, cured at ambient air, than for the FSH samples, cured in moist room. The development of the portlandite crystals in FSH samples can be explained by the presence of water in the pores of the samples. The water permits the Ca++ ions diffu- sion towards the matrix and then the growth of the portlandite the solution in the paste is always saturated with Ca++ ions and the voids are simply the free place for crystallization not only of portlandite but also of ettringite [28–30]. Portlandite peaks are weaker in the samples containing silica fume. That confirms a reaction between portlandite and silica, and thus the expected pozzolanic effect. The carbonation phenomenon is observed in all samples, regardless of the curing conditions, but is limited when samples are cured in moist room. When the relative humidity increases, the fraction of pores filled with water also increases and obstructs the diffusion of CO2 gas which is approximately 10,000 times smaller in liquid phase than in gaseous phase (in order of 10–12 m2/s, against 10–8 m2/s) [31].
However, the calcium carbonate is developed in the begin- ning near the surface of concrete element as the result of cal- cium hydroxide transformation. This transformation is accom- panied with increase of the content of solid phases that induces a decrease of the cement matrix porosity in the nearsurface.
This is mentioned by different authors [32, 34], who have attributed to the precipitation of calcium carbonate reducing porosity.
3.2 Microstructural observations to scanning electron microscope
The SEM observations are displayed in Figure 4 and con- cern matrixes made of CEM II/B-V 32.5. The first micrograph shows the aspect of the CemFil fiber before mixing: a regu- lar and smooth surface. FSH2 and FSH3 samples, conserved in moist room, present a strong degradation of the fibers, with attack spots observed along the surface. However, spots appear disseminated, the actual degradations are superficial and not homogeneous. It is noted that fibers are less damaged in the FSH3 samples owing to the silica fume additions. The observed degradation is marked either by erosion or notches. The sam- ples cured at ambient air, FCA2 and FCA3 remain intact.
[Q:quartz; P:portlandite; E:ettringite; CA: calcium carboaluminate; C: calcite; CSH: hydrate]
Fig. 3 X-ray diffraction patterns (Cu Kα)
Those observations confirm the results obtained by X-Ray diffraction. First, silica fume additions permit to protect the fibres due to the pozzolanic reaction that transforms portlan- dite into CSH and thus decreases the matrix alkalinity [34].
Second, the glass fibre appearance remains stable in dry envi- ronment: on the one hand, Ca++ ion diffusion is braked in this environment and the portlandite cannot develop. On the other hand, the carbonation, developed at the free surface of the samples, reduces the porosity.
In addition, the energy dispersive X-ray (EDX) analysis of the A–R glass fibre surface support the observed results. The localized chemical analysis realized on fibres body different
points (Figs. 5 and 6) shows a degradation of the silicic net- work of the glass fibre. In the FSH3, which contains the silica fume additions, there is no excessive degradation of the fiber body and the microanalysis shows silica (Si) contents higher than those detected on the fiber of the FSH2 sample. In FCA2 and FCA3 samples, which are conserved at ambient air, the energy dispersive X-ray (EDX) analysis show silica (Si) con- tents higher than those detected on the fibers of the samples conserved in moist room. This points that the conservation environment at ambient air, of the alkali-resistant glass fibres used in a cement matrix, does not favor the degradation of glass structure.
Fig. 4 SEM observations of the CemFil fiber (A.R.) before mixing and of the glass fibers in the CEM II/B 32·5 cement matrix, conserved in different envi- ronments
4 Conclusions
This paper has presented the experimental results of a study on the durability of alkali-resistant glass fibers reinforced cement mortar. Based on the results of this study, the follow- ing conclusions can be drawn:
If degradations seem to be inevitable, they are reduced by the use of fly ash containing cement (CEM II/B-V). It is also shown that silica fume additives reduce the fiber attack phe- nomena owing to the pozzolanic effect that transforms port- landite into C-S-H and decreases the porosity. However, using a low alkalinity cement reduces the amount of portlandite (Ca(OH)2) into the matrix, which results an impoverishment of hydroxyl ions production, thus a decreasing the pH of pore solutions and consequently the environment can be depleted (less basic). On the other hand, Ca++ ions that can be combined with silica fume to form the new hydrated calcium silicates are reduced. In addition, the inclusion of a mineral addition such as the silica fume has a slow reaction effect with calcium hydroxide (pozzolanic effect), whereas the kinetics of degra- dation of glass fibres occur earlier.
Under dry environment or low humidity, like a usual indoor building (case in ambient air), alkali-resistant fibres undergo surface modifications that look like disseminated spots. These degradations do not cast doubt on the structure stability of the glass fibre-reinforced cement.
On the contrary, the use of such materials in a humid envi- ronment (in moist room) has to be avoided. The stability of the structure would be endangered in spite of the fiber treatment and the silica fume use in the cement matrix.
Acknowledgement
The authors express their thanks to Joseph Le Lannic and Francis Goutefangeas, CMEBA Rennes1 University (France), for their assistance with SEM.
Fig. 5 Spectrometry X-ray analyses of the fibre body of GFRC cured in moist room
Fig. 6 Spectrometry X-ray analyses of the fibre body of GFRC cured at ambient air
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